1. Field of the Invention
The invention relates generally to door hinges and more particularly to door hinges for detecting whether a door is open or closed.
2. Description of the Prior Art
There are several situations where it is necessary to monitor whether a door is open or closed. Examples of such situations are turning a light on or off and managing building security to name a few. In some cases it is desirable to disguise the device that performs the monitoring in order to reduce the likelihood of tampering.
One approach for monitoring the state of a door uses a push button switch in a wall jamb. The push button is depressed by the doorjamb when the door is closed and is released otherwise. Although this approach is common in automobiles, its use for managing building security has been limited because the push button is exposed and therefore more likely to be tampered with and because it requires a custom installation of an additional part. The same issues limit the use of a reed relay and a magnet in place of the push button switch. Many attempts have been made over the years to improve upon these approaches by using a custom door hinge having a rotation sensitive switch hidden within the knuckles of the hinge. Unfortunately, such hinges have been costly due the extensive machining that has been required and they have not proven to be reliable. Another attempt has been made to resolve these issues by hiding the reed relay and magnet in cavities milled or cut into the jambs behind the hinge leaves. Unfortunately, the additional installation for this approach is costly because custom milling or cutting is required for the cavities. Further, it is not suitable for a steel hinge unless a non-magnetic insert is made in the hinge in the area covering the magnet and the switch. Making such insert increases the cost still more.
There remains to be a need for an inexpensive monitor hinge that is easy to install.
It is therefore an object of the present invention to provide an inexpensive monitor hinge using a magnet plug and a magnetic switch plug where the plugs have the installed appearances of hinge fastening screw heads. The monitor hinge of the present invention is inexpensive to install because the magnet plug and the magnetic switch plug are inserted through standard hinge fastening holes into round holes in door and wall jambs that may be drilled with a conventional drill bit.
It is another object of the present invention to provide a conversion kit having a magnet plug and a switch plug that may be used for converting standard hinges into monitor hinges.
Briefly, in a preferred embodiment, the monitor hinge of the present invention includes a hinge, a magnet plug having a magnet, and a switch plug having a magnetic field sensitive switch. The magnet plug and the switch plug replace two hinge fastening screws in mirror image positions when the hinge is open and mirror image juxtaposed positions when the hinge is closed. The magnet and switch plug housings have countersink heads with cross notches that seat in countersink hinge fastening holes and give the appearance of hinge fastening screws. The bodies of the magnet and switch plugs are cylindrical and have dimensions to pass through standard hinge fastening holes so that the plugs can be inserted into holes in the jambs that are drilled before or after the hinge is installed.